Heat treatment



Patented Apr. 8, 1941 HEAT TREATMENT Frank S. Badger, Jr.,

Haynes Stellite Indiana Kokomo, Ind., assignor to 0mm a. corporation ofNo Drawing. Application January 16, 1940, Serial No. 314,054

6 Claim.

This invention relates to a method of heat treating corrosion resistantnickel-base alloys containing molybdenum.

Nickel-base alloys containing about 10% to 40% molybdenum, which alloyssometimes contain one or more additional elements such as chromium,tungsten, vanadium, and iron in proportions up to or are resistant tocorrosion, and are widely used in the fabrication of articles andapparatus required to resist corrosion, for example, by hydrochloricacid. Ordinarily, alloys oi? this type are annealed by rapidly coolingthem irom a temperature which depends to some ex tent on the compositionof the alloy but is above about 1111.00 (3. and are used in the annealedcondition.

has been observed that when nickel -molybdenum alloys are exposed, eventor a short period of time, to temperatures within the range of about300 C, to 700 0., after having been anhealed, the corrosion resistanceof the alloys is seriously detrimentally affected. The effect ofexposure to this range of temperatures is well illustrated where a weldhas been made in such alloys. if a joint is made between two plates ofthe material, using as filler material an alloy of the same composition,the deposited weld metal is resistant to corrosive attack, but thoseportions or the plates on either side of the weld and'acljacent to itwhich have been heated by conduction to about 300 C. to 700 C. areseverely attached, while those portions of the plates remote trom theweld are resistant to attack. This effect of elevated temperatures onthe corrosion resisting properties of the alloy presents a seriousproblem.

It is an important object of this invention to treat alloys of the classdescribed to improve their resistance to corrosion and to decrease orprevent loss of corrosion resistance upon exposure to elevatedtemperatures. Another object of the invention is to provide a heattreatment which prevents excessive corrosion adjacent to welds in weldedarticles fabricated from alloys of this type. A further object is toheat treat such alloys to improve their physical properties not only atordinary temperatures, but also upon and after exposure to temperatureswithin the range of 300 C. to 700 C.

These and other objects are achieved by the present invention which isbased on the discovery that if fully annealed alloys which normally aredetrimentally affected by exposure to the detrimental temperature rangebetween 300 C. and 700 C, are heated to a temperature (intermediatebetween annealing temperatures and the detrimental temperature range)between about 900 C. and 1175 C. for about 2 to 72 hours they becomestabilized, their physical and corrosionresisting properties no longerbeing subject to deterioration on exposure to elevated temperatures.

Accordingly, the invention is a method of heat treating such alloys, forexample nickel-molybdenum alloys containing about 10% to 40% (preferably15% to 30%) molybdenum, with or without minor amounts of other alloyingelements, which comprises heating the alloys at an elevated stabilizingtemperature between 000 C. and ll75 0., for 2 to 72 hours. The heatedalloys may be cooled in air. Preferably, the temperature employed isbetween l020 C. and l065 0., and the material is maintained at suchtemperature for from 2 to 24 hours. When'a temperature near the lowerend of the range is chosen, the alloy or article being treated should beheated for a longer time than it a temperature near the higher end ofthe range is chosen, and relatively large articles should be maintainedat the treating temperature for a longer time than smaller articles.When the alloys are stabilized by sufficient exposure to stabilizingtemperatures, the effect on corrosion resistance of either previous orsubsequent exposure to tem peratures within the range of 300 C, to 700C. is counteracted.

The invention also includes a corrosion-resistant welded articlefabricated from a nickelbase alloy containing molybdenum, such articlebeing resistant to preferential corrosion in the weld zone, itscorrosion resistance being substantially uniform over the welded surfaceand the surface adjacent the weld zone.

Tests have been made on nickel-molybdenum alloys of several differentcompositions in which the specimens under test were exposed to boilingconcentrated hydrochloric acid. Certain of the specimens were tested inthe annealed condition. Others were tested after they had been annealedand then exposed to temperatures within the damaging range. Still otherspecimens were annealed, given the heat treatment of the invention, andthen subjected to temperatures between 300 C. and 700 C. It was found bysuch tests that alloys which had not been given the heat treatment ofthe invention but had been exposed to elevated temperatures sulfered aloss in weight of as much as nine times the loss in weight of similaralloys not exposed to those temperatures, while alloys which had beenheat treated and then subjected to temperatures between 300' C. and 700C. were fully as corrosion resistant as alloys which had not beensubjected to those temperatures. Further, the corrosion-resistance ofheat treated specimens which had not been subjected to elevatedtemperatures before test was better than that of specimens not heattreated. The results of these tests are given in Table I.

TAnLr: I

Corrosion rates, mg./sq. cm./hr. in boiling concentrated HCLafterpreliminary heat treatment and subsequent holding for one week at tem-Tensile tests of samples of a nickel-base alloy containing 20%molybdenum and 20% iron conducted at 500 C., after the test pieces hadbeen held at that cated that the physical properties of such alloys areconsiderably improved by the heat treatment of the invention. Results ofsuch tests are shown in Table II. Specimen A was annealed by waterquenching from 1160 C. Specimen-B was annealed in the same manner andwas then heated at 1020" C. for 6 hours and air cooled. Both specimenswere then held for one week at 500 C.

TABLE II Results of tensile tests conducted at 500 C. of Ni-base alloyscontaining 20% M and 20% Fe Yield Tensile Reduction point strengthElongano of area P. s P. s. l. Percent Percent Specimen A. 40, 000 76,500 20. 2 Specimen B 37.000 98, 000 29. 5 26. 8

Itis an important advantage of the invention that articles may befabricated by welding from nickel-base alloys containing molybdenumwithout the danger of such articles sufiering a loss of corrosionresistance in the vicinity of the welds. When apparatus is to be madefrom such alloys by welding, it is preferable first to fabricate theapparatus and then to heat treat it as a whole. However, where this isimpracticabl the various parts of the article or apparatus may be heattreated before they are welded, and, in such case, no heat treatment ofthe welded article is necestemperature for one week, indisary. If suchapparatus is fabricated-from annealed alloys, it is unnecessary toanneal the apparatus.

While in the above examples of the. invention, specific reference hasbeen made to nickelmolybdenum alloys, the presence of chromium ortungsten or both does not substantially alter the effect of the heattreatment described. Chromium may be included in such alloys up to aboutand tungsten may also be included up to about 7%. If chromium ispresent, the maximum proportion preferable is 15%, and if tungsten ispresent, the maximum proportion preferable is 5%. Alloys containingchromium or tungsten or both should be heat treated near the higherlimits of the temperature range defined. Iron may be included in thealloys as a diluent in proportions up to about preferably between 5% and20%. Minor proportions of manganese, silicon, vanadium, carbon,zirconium, tantalum, columbium, and titanium may also be included in thealloys with little effect on the benefit derived from the heattreatment.

I claim:

1. Method of treating nickel-base alloys containing 10% to v molybdenumwhich comprises heating such alloys, after they have been fullyannealed, at a temperature between 900 C. and 1175 C. for about 2 to 72hours.

2. Method of treating nickel-base alloys containing 10% to 40%molybdenum which comprises heating su-ch alloys, after they hav beenfully annealed, at a temperature within the range of 1020 C. to 1065 C.for about 2 to 24 hours.

'3. Method of treating nickel-base alloys containing 10% to 40%molybdenum which comprises fully annealing such alloys and then heatingthe annealed alloys at a stabilizing temperature between 900 C. and 1175C. for about 2 to 24 hours.

4. Method of treating nickel-base alloys containing 10% to 40%molybdenum which comprises rapidly cooling such alloys from a temperature above 1100 C. and then heating the alloys at an annealingtemperature within the range of 1020 C. to 1065 C. for about 2 to 24hours.

5. A corrosion-resistant welded article fabricated from a nickel-basealloy containing 10% to 40% molybdenum, said article being in thecondition produced by heating within the range of stabilizingtemperatures between 900 C, and 1175 C., and the corrosion resistance ofsuch article being substantially uniform over the Welded surface and thesurface adjacent the weld zone.

6. A welded article iarbricated from a nickelbase alloy containing about10% to 40% molybdenum, such article being in the condition produced byheating at stabilizing temperature-s within the range of 900 C. to about1065 C. and being resistant to preferential corrosion at the areasadjacent the weld zones when subjected to corrosive media which normallywould cause corrosion to take place at a more rapid rate at such areasthan at areas remote from the weld zones.

FRANK S. BADGER, JR.

